The present invention relates generally to data communications. The invention more particularly relates to timing recovery techniques used in data receivers which have automatic and/or adaptive equalizers.
Accurate reception of high-speed data signals transmitted over a bandlimited channel with unknown transmission characteristics requires the use of an automatic equalizer. The equalizer, which is resident in the receiver portion of a data set, or modem, is generally in the form of a transversal filter. Samples of the received data signal, referred to herein as line samples, are formed at a predetermined sampling rate. In a so-called T/2 equalizer, for example, the line samples are formed at twice the transmitted symbol rate. The line samples are applied to the transversal filter, in which each is multiplied by respective one of a queue of coefficients. The resulting products are added together and, if necessary, demodulated to generate a baseband signal, referred to herein as an equalizer output. The value of each equalizer output is used as the basis for forming a decision as to the value of a respective transmitted data symbol.
In addition, an error signal is formed equal to the difference between each equalizer output and a reference signal which represents its respective data symbol. In the so-called adaptive type of automatic equalizer, in particular, the reference signal is derived from above-mentioned decision. The error signal is used to update, or adapt, the transversal filter coefficient values in such a way as to minimize a measure of the channel-induced distortion--assumed to be primarily intersymbol interference--in the equalizer outputs.
An important equalizer operating parameter, in addition to the rate at which the line samples are formed, is their time occurrence with respect to the received signal. This parameter is referred to as the timing epoch. The coefficient values subsisting in the equalizer at any given time will yield accurate equalization only if the received signal is sampled at or near a particular set of time points on the received signal, i.e., only if it is sampled with the appropriate timing epoch. The frequencies of the transmitter and receiver clocks invariably differ from one another, if only by a very small amount. Over time, this frequency difference, if not compensated for, would cause the received signal to be sampled further and further away from the appropriate time points, i.e., with an increasingly erroneous timing epoch. As long as the sampling frequency is high enough, the equalizer does have the ability to compensate for this clock frequency difference (as long as it is not too large) via the coefficient update process. This is not an effective long-term solution, however, because the distribution of coefficient values will eventually become skewed to one end of the coefficient queue and equalizer performance will degrade sharply.
To deal with this problem, the receiver is conventionally provided with a so-called timing recovery circuit. The timing recovery circuit determines whether the line samples are being formed earlier (later) than they should be and, in response, adjusts the phase of the line sample forming circuitry such that the line samples are formed a little later (sooner) than they otherwise would. This phase adjustment process is referred to as retarding (advancing) the receiver timing or, alternatively, as retarding (advancing) the sampling phase. The amount by which the receiver timing is advanced or retarded is referred to herein as the timing adjustment increment.
A commonly used timing recovery technique is so-called envelope-derived timing recovery, disclosed, for example, in the Bell System Technical Journal, Vol. 54, p. 569 et seq, March, 1975. This technique extracts a symbol-rate tone from the received signal and uses the phase of that tone to control receiver timing. Envelope-derived timing recovery performs satisfactorily for many applications. In some situations, however--such as a narrow rolloff system--the recovered tone may be so weak that accurate timing recovery is not possible when random data is being received.
An alternative timing recovery technique, referred to herein as "coefficient tracking," controls receiver timing as a function of coefficient distribution within the queue. One such technique is disclosed in U.S. Pat. No. 4,004,226, issued Jan. 18, 1977 to S. U. H. Qureshi et al. A particular coefficient--typically at or near the queue midpoint--is designated as the one which should have the largest magnitude. Periodically, e.g., in each symbol interval, the coefficient which actually has the largest magnitude is identified. If it is found to be the designated coefficient, no timing adjustment is made. Otherwise, the receiver timing is advanced or retarded, as appropriate, by a predetermined fixed timing adjustment increment such that subsequent coefficient adaptation over a number of symbol intervals causes the magnitude of the designated coefficient to ultimately become the largest.
Another coefficient tracking timing recovery technique is disclosed in the co-pending U.S. patent application of R. D. Gitlin et al, Ser. No. 185,016 filed of even date herewith. In accordance with that technique, the coefficient queue is divided into front and back portions, only. That is, every coefficient is in one or the other portion and no one coefficient is designated to have the largest magnitude. When a reference coefficient, such as the largest coefficient, is found in the front portion, receiver timing is retarded. If it is found in the back portion, receiver timing is advanced. This causes the location of the reference coefficient to alternate (over a large number of symbol intervals) between two coefficients, one on either side of the boundary between the front and back portions of the coefficient queue.
The use of a timing adjustment increment which is fixed may require a design compromise. In particular, a fixed timing adjustment increment should be large enough to ensure accurate timing recovery when the transmitter/receiver clock frequency difference is at some predetermined anticipated maximum, i.e., the worst case. On the other hand, the equalizer performs best when the timing adjustment increment is just barely large enough to compensate for the frequency difference which actually obtains.